1. Field of the Invention
This invention generally relates to an injection molding method and apparatus, and more particularly to a gate valve arrangement which includes insulation to thermally segregate a heated runner housing from an adjacent cooled mold cavity.
2. The Prior Art
Various valving arrangements have been proposed in the prior art for regulating the flow of heated thermoplastic material from a heated runner housing to the sprue passageway of a cooled mold assembly. For the most part, the prior art includes two primary types of valving arrangements: (1) a slidable gate valve, shown for example by U.S. Pat. Nos. 3,288,903, 3,799,723, 3,787,159, 3,387,324 and 3,694,529; and (2) an axially reciprocal pin valve shown for example by U.S. Pat. Nos. 3,023,458, 3,530,539, 2,878,515, and 3,296,353.
Typical problems in this area of technology have included drool from the sprue passageway, cold slugs formed in the runner system as a result of heat being lost by conduction to the cooled mold assembly, and warm spots in the mold assembly due to heat conduction, causing longer cycle time to assure complete curing of the molded part.
Other problems exist due to the nature of certain plastic materials. For example, polyethylene terephthalate, being readily crystallizable when cured and solidified, is sensitive to nonuniformly cooled molds and to bushing designs where the sprue opening is too small, causing strong shear fields in the material.
Prior attempts to solve these problems have not been totally successful. For example, U.S. Pat. No. 3,741,704 discloses the use of insulation material between a cooled mold and a heated runner housing, but does not prevent the conduction of heat between these two mold assembly parts through the gate valve. Other patents such as 2,928,125 and 3,579,620 generally disclose the use of insulation but not in the context of the present invention.
The present invention overcomes all the previously discussed prior art problems by effectively utilizing insulating material in conjunction with valving structure to thermally separate the heated and cooled zones in the injection molding process.